1. Field of the Invention
This invention relates to methods of preventing corrosion and to apparatus for use in such methods.
2. Introduction to the Invention
It is well known to protect an electrically conductive substrate from corrosion by establishing a potential difference between the substrate and a spaced-apart electrode. The substrate and the electrode are connected to each other through a power supply of constant sign (DC or rectified AC) and the circuit is completed when electrolyte is present in the space between the substrate and the electrode. In most such impressed current systems, the substrate is the cathode (i.e. receives electrons). However, with substrates which can be passivated, e.g. Ni, Fe, Cr and Ti and their alloys, it is sometimes also possible to use impressed current systems in which the substrate is the anode. In both cathodic and anodic systems, the substrate is often provided with a protective insulating coating; in this case the impressed current flows only through accidentally exposed portions of the substrate. If the system is to have an adequate life, the electrode must not itself be corroded at a rate which necessitates its replacement; this is in contrast to the "sacrificial anodes" which are used in galvanic protection systems. The electrode must also have a surface which is not rendered ineffective by the current passing through it or by the electrochemical reactions taking place at its surface, such as the evolution of chlorine gas.
The electrode and the power supply must be such that the current density at all points on the substrate is high enough to prevent corrosion but not so high as to cause problems such as damage to the substrate (eg. embrittlement) or disbonding of a protective coating on it. The power consumption of the system depends inter alia on the distance between the various parts of the substrate and electrode. In view of these factors, the theoretically best type of electrode is one which has a shape corresponding generally to the shape of the substrate and which is relatively close to all points on the substrate. Such an electrode is referred to herein as a "distributed electrode". Distributed electrodes have been provided in the past, for example, by a layer of conductive paint which is coated over an electrically insulating coating on the substrate, or by a platinum-coated wire placed adjacent to the substrate (usually inside a pipe). However, the known distributed electrodes suffer from serious practical disadvantages. Conductive paints require careful, craft-sensitive application of the paint layer and of the insulating layer; and even when the layers are correctly applied, the paint (whose thickness is less than 200 microns, usually less 100 microns) can easily be damaged either by mechanical abrasion or by blistering or peeling due to passage of current. Furthermore, unless the paint is of very low resistivity (which makes it difficult to apply and/or very expensive and/or more liable to damage), either the size of the substrate is very limited or there must be bus bars between the paint and the insulating layer. Such bus bars, especially if exposed as a result of damage to the paint, are liable to corrosion. The disadvantages of platinum-coated wires are likewise numerous. Platinum is very expensive (which is of course why platinum-coated wires, rather than pure platinum wires, are used) and platinum coatings are very easily damaged, eg. by flexing of the wire. The use of platinum-coated wires is, therefore, restricted to situations in which such damage can be minimized. In addition, it is essential that the core of the wire, if exposed, is not liable to corrosion, and this further increases the cost of the electrode. In practice, platinum-coated wires comprise a core of titanium or niobium-coated copper.
Because of the difficulties associated with distributed electrodes, most practical impressed current corrosion protection systems make use of a plurality of discrete electrodes which are spaced apart at some distance from the substrate. Typically, the anodes are rigid rods which are composed of (a) graphite or (b) a thermoset resin or other rigid matrix which is highly loaded with graphite or other carbonaceous material. Because of the distance between the electrodes and the substrate, large power supplies are often needed and interference from other electrical systems (including other corrosion protection systems) is common. In addition, the high current density at the electrode can give rise to problems, eg. in dispersing gases generated by electrochemical reactions at the surface of the electrode.